1. Field of the Invention
The invention relates to a method for mounting a semiconductor chip provided with bump electrodes on a circuit board with wiring patterns formed thereon, and particularly, to a method for fixing a semiconductor chip to the circuit board with the use of an anisotropic conductive adhesive while connecting the bump electrodes of the semiconductor chip to the wiring patterns on the circuit board.
2. Description of the Related Art
A method for mounting a semiconductor chip (device) for driving liquid crystals on, for example, a glass substrate of a liquid crystal panel cell by use of an anisotropic conductive adhesive has already been put to commercial use.
Such a conventional method for mounting the semiconductor chip is described hereinafter with reference to FIG. 5 showing a plan view, FIG. 6 showing a sectional view taken along the line A--A of FIG. 5, and FIGS. 7-1 to 7-4 sectional views similar to FIG. 6, showing respective steps in the method for mounting the semiconductor chip.
As shown in FIG. 5, a liquid crystal display has a construction wherein wiring patterns 15 for outputting signals to the display thereof are formed on blank spaces 16 of a first substrate 11 and a second substrate 12, respectively, making up a circuit board of a liquid crystal panel cell, so that a plurality of semiconductor chips 13 for driving liquid crystals can be mounted on the wiring patterns 15.
As material for the wiring patterns 15, a transparent and electrically conductive film such as an indium tin oxide (ITO) film, tin oxide film, or the like, is used.
As shown in FIG. 6, the semiconductor chip 13 to be mounted on the blank spaces 16 of the substrates 11, 12, with the wiring patterns 15 formed thereon, is adhered thereto, respectively, by means of an anisotropic conductive adhesive 18.
The anisotropic conductive adhesive 18 is an insulating epoxy-based adhesive mixed with metal particles of silver, solder, or the like, 5 to 10 .mu.m in grain size, or mixed with electrically conductive particles 18a such as plastic particles rendered electrically conductive by plating the surface thereof with gold, or the like.
The wiring patterns 15 are rendered electrically continuous with bump electrodes 14 provided on the semiconductor chip 13, opposite to the wiring patterns 15, respectively, by interposing therebetween the electrically conductive particles 18a contained in the anisotropic conductive adhesive 18.
An anisotropic conductive film (ACF) 180 sandwiched between a base film 181 and a cover film 182 as shown in FIG. 11 is available as the anisotropic conductive adhesive 18 in the form of a film. The ACF 180 is an adhesive layer composed of thermosetting epoxy resin, containing a plurality of the electrically conductive particles 18a.
As shown in FIG. 12, the electrically conductive particles 18a are spherical in shape and on the order of 5 .mu.m in diameter. A gold plated layer 18a2 is formed on the surface of a plastic core 18a1 thereof, and the surface of the gold plated layer 18a2 is further covered with an insulation layer 18a3.
Accordingly, the electrically conductive particles 18a are insulated from each other within the ACF 180. The ACF 180 sandwiched depthwise between electrically conductive members, when heated and compressed, becomes electrically conductive due to destruction of the insulation layer 18a3 of the electrically conductive particles 18a. However, conduction of electricity does not occur in the lateral direction of the ACF 180 because the insulation layer 18a3 of the electrically conductive particles 18a present in lateral regions is not destroyed.
Steps in the conventional method for mounting a semiconductor chip are described hereinafter with reference to FIGS. 7-1 to 7-4.
In Step 1, as shown in FIG. 7-1 the anisotropic conductive adhesive 18 is transferred to and disposed on portions of the blank spaces 16 of the substrate 12, where the semiconductor chip 13 is to be mounted.
For example, after the cover film 182 shown in FIG. 11 is peeled off and the ACF 180 is pasted on the substrate 12, the base film 181 is peeled off.
Subsequently, in Step 2, as shown in FIG. 7-2, after aligning the bump electrodes 14 of the semiconductor chip 13 with the wiring patterns 15 facing the bump electrodes 14, the semiconductor chip 13 is disposed on the substrate 12, with the anisotropic conductive adhesive 18 interposed therebetween.
In Step 3 as shown in FIG. 7-3 thereafter, by use of a heating and pressing jig 19 provided with a heater 19a built therein, the semiconductor chip 13 is thermally press-bonded to the second substrate 12 by heating while applying pressure, hardening the anisotropic conductive adhesive 18.
In Step 4 as shown in FIG. 7-4 with the anisotropic conductive adhesive 18 hardened in the preceding step, the semiconductor chip 13 is adhered to the substrate 12, thereby clamping a plurality of the electrically conductive particles 18a between the bump electrodes 14 of the semiconductor chip 13 and the wiring patterns 15 over the substrate 12, opposite thereto, respectively, with the result that the respective bump electrodes 14 are rendered electrically continuous with the respective wiring patterns 15.
As the anisotropic conductive adhesive 18 is an insulating epoxy-based adhesive with the electrically conductive particles 18a dispersed therein, or as with the case of ACF 180 shown in FIGS. 11 and 12, the spherical surface of the respective electrically conductive particles 18a is covered with the insulation layer 18a3, the electrically conductive particles 18a, other than those clamped between the bump electrodes 14 and the wiring patterns 15, are insulated from each other. Accordingly, there is no possibility of short-circuit occurring between the individual bump electrodes 14 themselves, or between the independent wiling patterns 15.
In the conventional method for mounting the semiconductor chip described above, however, air bubbles involving volatile constituents (diluent, moisture, and the like) contained in the anisotropic conductive adhesive 18, air, and the like are formed between the semiconductor chip 13 and the substrate 12 when hardening the anisotropic conductive adhesive 18 by heating.
Thus, as shown in FIG. 8, air bubbles 21 are formed on the surface of the epoxy-based adhesive material of the anisotropic conductive adhesive 18 between the semiconductor chip 13 and substrate 12, where adhesion is to take place.
As a result, the epoxy based adhesive material of the anisotropic conductive adhesive 18 cannot be fully filled between the semiconductor chip 13 and substrate 12, impairing adhesion strength. This has caused a problem of electrical continuity between the bump electrodes 14 and wiring patterns 15 being disrupted at times due to exfoliation of the semiconductor chip 13 from the substrate 12.
Further, after the semiconductor chip 13 is press-bonded to the substrate 12 by heating, the semiconductor chip 13 and substrate 12 will be found in a condition wherein thermal strain occurs therebetween.
In comparing the thermal expansion coefficient of, for example, a borosilicate glass substrate often used for the substrate 12 of the liquid crystal panel cell with that of the semiconductor chip 13 composed primarily of silicon, the glass substrate has a higher thermal expansion coefficient. Accordingly, when the temperature of the anisotropic conductive adhesive 18 declines to room temperature after the semiconductor chip 13 has been press-bonded to the substrate 12 by heating and the anisotropic conductive adhesive 18 has been hardened, the difference in thermal expansion coefficient between the semiconductor chip 13 and substrate 12 causes a difference in shrinkage to occur between materials making up respective members.
FIG. 9 shows the relationship between amounts of thermal expansion and heating temperatures with respect to a borosilicate glass substrate and a semiconductor chip. The thermal expansion coefficient of the borosilicate glass substrate, .alpha. glass, is expressed as follows: EQU .alpha. glass=51.times.10.sup.-7 /.degree.C.
On the other hand, the thermal expansion coefficient of the semiconductor chip composed primarily of silicon, .alpha. IC, is expressed as follows: EQU .alpha. IC=24.2.times.10.sup.-7 /.degree.C.
That is, the first and second substrates 11, 12 shown in FIGS. 5 to 7, which are glass substrates, have thermal expansion coefficients twice as high as that of the semiconductor chip 13.
When a temperature difference of .DELTA.T is applied to a material 1 meter long and having thermal expansion coefficient .alpha., elongation L (m) of the material is generally found by the following formula: EQU L=.alpha..times.1.times..DELTA.T.
Assuming that the anisotropic conductive adhesive 18 is cured at 210.degree. C., the semiconductor chip 13 needs to be heated to a temperature on the order of 250.degree. C. Further, the duration of press-bonding is in the range of 5 to 10 seconds.
With the duration of press-bonding ranging from 5 to 10 seconds, the temperature on the side of the substrate 12 rises to only around 100.degree. C.
The elongation occurring to the substrate (glass substrate) 12 and the semiconductor chip 13, respectively, is calculated hereinafter assuming that room temperature is 20.degree. C.
Assuming further that a semiconductor chip has a side 15 mm long, and the elongation caused by thermal expansion occurs bisymmetrically, calculation of the elongation is made for one half of respective members on one side only.
Then, the elongation of the substrate 12 is found by the following formula: EQU (15 mm.div.2).times.51.times.10.sup.-7 .times.(100.degree. C.-20.degree. C.)=0.0030600 mm.
On the other hand, the elongation of the semiconductor chip is found by the following formula: EQU (15 mm.div.2).times.24.2.times.10.sup.-7 .times.(250.degree. C.-20.degree. C.)=0.0041745 mm.
Accordingly, the discrepancy in the elongation between the substrate 12 and the semiconductor chip 13 amounts to the value given below, indicating that the semiconductor chip 13 is further elongated by about 1 .mu.m: EQU 0.0041745-0.0030600 mm=0.0011145 mm.
If respective materials are left as they are after the temperatures thereof are allowed to come down to room temperature at 20.degree. C. with such a difference in elongation therebetween as described remaining, thermal strain will occur on bonded surfaces of the semiconductor chip 13 and substrate 12 (more specifically, between the bump electrodes 14 and the wiring patterns 15, and between the semiconductor chip 13 and substrate 12) due to a difference in shrinkage therebetween, causing a problem of exfoliation.
FIG. 10 shows the relationship between temperatures of the borosilicate glass substrate and strain in the semiconductor chip for driving liquid crystals when heated to 250.degree. C. in relation to the glass substrate.